This invention relates generally to debarking of logs for pulp manufacture and more particularly to an apparatus and method for providing steam to the outside surface of the logs during their passage through the debarking drum, thereby heating the bark and improving debarking performance.
During winter, it is common for pulp logs which are stored outdoors prior to debarking to be in a frozen condition. This makes debarking difficult, and since it increases the time necessary for debarking to be accomplished, it also reduces the debarking throughput capacity of the mill.
Steaming frozen pulp logs is known to ease the removal of bark. A number of different steaming methods have been used. One steaming method for continuous drum debarking is to feed the frozen pulp logs through a stationary array of steam nozzles immediately prior to their entry into the debarking drum. This method has the disadvantage of being unable to preheat all surfaces of the feed logs during the debarking process. This is so because the logs in the feed chute are steamed as they pass the steam nozzles while sliding toward or into the debarking drum with little or no tumbling motion. The limited exposure of the log surface to the steam results in non-uniform thawing and a temporary surface warming which, in many instances, results in glazing, or icing, of the bark surface and, consequently, non-uniform debarking effectiveness.
Another method of steaming logs for debarking is to introduce the steam into the debarking drum in a substantially radial direction. This is accomplished by means of a compartmented annular steam distribution ring, in conjunction with a circumferential sliding valve which provides steam pressure through sequential steam ports in the drum wall. Commonly, the steam from the distribution ring compartments is distributed bidirectionally via hollow longitudinal ducts and thence into the drum through radial openings.
All steaming or thawing techniques presently employed provide some degree of success, but all also are subject to varying degrees of inadequacy. Out of roundness of the annular steam distribution ring can cause loss of steam pressure at entry into the drum interior and also leakage between the sliding valve and the ring as can wear of the mating surfaces of these two members. Steaming or water spraying in the log feed chute produces massive quantities of polluted water and may permit re-icing of some logs before they enter the debarking drum.
Also, because of its orientation, the circumferential sliding valve provides an opportunity for entrapment of foreign material between the valve surface and the annular steam distribution ring. This compromises the seal integrity at the contact interface between these two members. In addition, wear of the circumferential sliding valve will prevent closure of the clearance gap between the ring and valve necessary to minimize steam leakage and loss of steam pressure. Drain back of condensate from the ducts within the drum will find its way into the steam distribution ring and from there into the steam valve. This necessitates a drain off provision which, in a steam pressurized valve, may make it difficult to maintain desired steam pressure. Finally, introduction of the steam through the steam distribution ring at an intermediate location along the length of the debarking drum makes it difficult to ensure desired distribution of steam in both longitudinal directions within the debarking drum. This is a consequence of the steam having to travel in two opposite directions through the ducts from the steam distribution ring.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.